Trace amounts of chelating agents in electroplating rinse water do not have a single effect in a low-temperature evaporation system. During the dilution stage, they delay the crystallization and precipitation of inorganic salts; however, once the solution enters the high-concentration stage, they actually induce and accelerate scaling on heat exchange surfaces. It is commonly observed on-site that the system operates relatively cleanly in the early stages, but scaling suddenly worsens once the concentration reaches high levels.

Since the feed solution is rinse water with relatively low salinity, chelating agents such as EDTA, citric acid, and tartaric acid form water-soluble chelate ions with heavy metal ions like copper, nickel, and zinc. These chelates bind the metal ions within the solution, reducing the effective concentration of free metal ions in the system. When the salt solution reaches the supersaturation level of ordinary salts but has not yet met the conditions for complex precipitation, inorganic salts do not readily form crystal nuclei, and crystal precipitation is delayed. This is the stage where salt crystallization is delayed. During this stage, no obvious crystallization is visible inside the chamber, and granular salt scale is unlikely to form on the heat exchange plates. Many operators mistakenly believe that chelating agents provide scale inhibition. However, low-temperature evaporation merely removes water; chelating agents do not volatilize with the steam but remain in the mother liquor, where they continuously accumulate. As the concentration ratio increases, both the salinity of the mother liquor and the concentration of chelating agents rise simultaneously, causing the operating conditions to reverse.

As the concentration ratio continues to rise and the ionic strength increases significantly, the solution’s pH also shifts, disrupting the coordination equilibrium between metals and chelating agents. Some complexes become less stable and begin to dissociate, releasing the heavy metal ions they had encapsulated back into the liquid phase. This results in a brief spike in free metal ion concentration within the system, leading to the rapid formation of hydroxides and microcrystals of basic salts. Additionally, some chelating agent–metal composite chelates have limited solubility; when concentrated to a critical concentration, they precipitate directly in the form of composites. These precipitates are not conventional inorganic hard salt crystals but rather viscous, gel-like organic–inorganic composite deposits. The adhesion of these colloidal deposits is far greater than that of ordinary salt crystals; they adhere very easily to heat exchange surfaces, directly forming a scale-forming substrate. Subsequent inorganic salts can easily continue to deposit and grow on this substrate, causing the scale layer to thicken rapidly—which is the root cause of accelerated scaling during the late stages of concentration.

The operating temperature in low-temperature, vacuum evaporation is not high enough to completely break down and decompose the complexing agents; thus, these agents accumulate continuously throughout the entire concentration process and do not dissipate on their own. Under circulating flow conditions, fine colloidal microcrystals move with the feed solution, resulting in a relatively gradual rate of scale formation; however, once localized low flow velocities occur and the boundary layer stagnates on the heat exchange plate surface, colloidal complexes preferentially accumulate and adhere within the boundary layer, gradually spreading out to form a continuous scale layer. Unlike crystallization of ordinary inorganic salts, the scale formed by these complexes contains organic matter and has a relatively tough texture. It is difficult to remove through hydraulic flushing alone, and conventional water washing is ineffective; acid washing is required for removal.

There are two additional associated phenomena observed in the field. First, there is an increased risk of foaming and carryover. Since most complexing agents contain organic functional groups, their concentration increases the strength of the foam film in the feed solution; negative-pressure boiling further facilitates the formation of fine, dense foam, exacerbating carryover in the demister. Second, the process of shutdown and standstill further exacerbates scaling. Without flow turbulence after shutdown, the complex compounds are more likely to settle and adhere to the heat exchange surfaces, and scaling becomes increasingly pronounced after multiple start-stop cycles.

Misjudgments are common under actual operating conditions. If initial operation proceeds smoothly, pre-treatment to remove complexing agents is often overlooked; it is not until high concentration ratios are reached that scaling becomes apparent. Many people directly attribute this to salt content, overlooking the underlying cause of the continuous accumulation of trace amounts of complexing agents. If the rinse water contains complexing agents and reliance is placed solely on evaporation without performing complex-breaking pretreatment upstream, stubborn scaling will still occur during the late stages of concentration—even if the equipment’s heat transfer efficiency and circulation flow rates fully meet specifications. In summary, complexing agents inhibit crystallization during the low-concentration rinse stage but accelerate scaling after high-concentration enrichment; the risks are most pronounced during the late stages of concentration and the mother liquor discharge phase.